Cassini Nears Phoebe

NASA News Release

2004 June 9

The most complex interplanetary mission ever launched is about to meet one of the
solar system's enigmatic moons. Cassini will fly by Phoebe, Saturn's largest outer
moon, on Friday. The closest approach is at approximately 4:56 p.m. EDT.

A final trajectory correction maneuver is scheduled for June 16. On arrival date,
June 30, Cassini will become the first spacecraft to orbit Saturn. Once in orbit it
will conduct an extensive, four-year tour of the Saturn system, including its majestic
rings and many known moons.

"The arrival date and trajectory to Saturn were specifically selected to accommodate
this flyby, which will be the only opportunity during the mission to study Phoebe at
close range," said Dave Seal, mission planner for the Cassini-Huygens mission at
NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. "Phoebe's orbit is simply too
far from Saturn, at almost 13 million kilometers (about 8 million miles), nearly four
times as far as the next closest major satellite, Iapetus. A later encounter is not
feasible."

"The last time we had observations of Phoebe was by Voyager in 1981," said Dr.
Torrence Johnson, former Voyager imaging team member, Galileo project scientist and
Cassini imaging team member. "This time around, the pictures of the mysterious moon
will 1,000 times better, as Cassini will be closer," he said. Voyager 2 captured
images of Phoebe from about 2.2 million kilometers (about 1.4 million miles) away.

Cassini will obtain images from a mere 2,000 kilometers (about 1,240 miles) above the
moon's surface. Cassini will also collect spectroscopic and radar data that could
decipher the composition and origin of this distant moon. Cassini's Phoebe images,
already twice as good as returned by Voyager 2, show large craters and variations in
surface brightness.

"Phoebe will be heavily cratered in the higher resolution images we expect to see in
the next few days," said Dr. Peter Thomas, a member of the imaging team and a senior
research associate at Cornell University, Ithaca, N.Y., who specializes in studies of
small satellites. "The hints of different brightness also suggest the highest
resolution images, several hundred times better, will show a variety of materials," he
said.

Discovered in 1898 by American astronomer William Henry Pickering, Phoebe is of great
interest to scientists. "With the instruments Cassini carries, we might learn more
about Phoebe's internal structure and composition. What we have are many unanswered
questions: Did it ever melt? Does it have evidence of past interior melting? Was it
ever an icy body? Why is Phoebe in such an odd orbit?" said Dr. Dennis Matson, project
scientist for the Cassini-Huygens mission at JPL.

Phoebe has a diameter of 220 kilometers (about 136.7 miles), which is equal to about
one-fifteenth of the diameter of Earth's moon. Phoebe rotates on its axis every nine
hours and 16 minutes, and it completes a full orbit around Saturn in about 18 months.
Its elliptical orbit is inclined approximately 30 degrees to Saturn's equator.
Phoebe's retrograde orbit means it goes around Saturn in the opposite direction of the
larger interior Saturnian moons. Previous ground-based observations have shown water
ice present on its surface.

Phoebe is also unusual as it is very dark. It reflects only six percent of the
sunlight it receives. Phoebe's darkness and retrograde orbit suggest it is most likely
a captured object. A captured object is a celestial body caught by the gravitational
pull of a much bigger body, generally a planet. Some scientists believe Phoebe might
even be an object from the outer solar system, similar to the objects found in the
Kuiper Belt. The Belt is a collection of small icy bodies beyond Pluto that were never
drawn together by gravity to form a planet.

"The dark and odd-shaped Phoebe may be a piece of the building blocks from which some
of the planets formed," said Dr. Bonnie Buratti, scientist on the Cassini-Huygens
mission at JPL. "It might hold clues about the early formation of our solar system."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space
Agency and the Italian Space Agency. JPL, a division of the California Institute of
Technology, Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space
Science, Washington. JPL designed, developed and assembled the Cassini orbiter. For
the latest images and information about the Cassini-Huygens mission on the Internet,
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